This invention relates to a process and apparatus for the liquefaction of a gas, and more particularly relates to a process and apparatus for the liquefaction of natural gas primarily comprised of methane, and including heavier hydrocarbons such as ethane, propane, butane and the like. Components heavier than the C.sub.4 fraction are a major problem in any liquefaction system, since such components freeze at the low temperatures thereby fouling the liquefaction equipment. Additionally, such heavier components may be more valuable as gasoline or other fuels.
There are many reasons for reducing natural gas to a liquefied state. One of the main reasons for liquefying natural gas is the resultant reduction of the volume of the gas to about 1/600 of the volume of natural gas in the gaseous state. Such a reduction in volume permits the storage and transportation of liquefied natural gas in containers of more economical and practical designs. Another important reason is the transportation of liquefied natural gas from a source of plentiful supply to a distant market where the source and supply may not be efficaciously joined by pipe lines, such that the transportation in the gaseous state would be uneconomical.
With the discovery of gas and oil in the Canadian arctic islands to the north of mainland Canada, much thought has been given to devising ways of bringing these commodities, particularly natural gas, to the user markets. While alternate methods to gas pipeline transportation from Prudhoe and the Mackenzie River delta are feasible, the problem of natural gas transportation to the user markets is even more formidable from the fragmented island chain in the Canadian high north, where the initial expense of a pipeline system is substantial for an uncertain capacity and reserve. The erection of a large LNG plant on Ellesmere island, for example, would be most difficult in view of the many imponderables.
Liquefaction systems may be classified according to the method of refrigeration employed to separate the gaseous mixture. There are three common methods of producing the necessary refrigeration, namely (i) vaporization of a liquid refrigerant, (ii) use of the Joule-Thompson effect; and (iii) a Claude cycle, using expanders and compressors. The present invention is a combination of methods (ii) and (iii).
The vaporization method (i) is a simple process wherein the refrigerant fluid is condensed by compression and cooling. The condensed liquid is throttled to a lower pressure whereby a portion flashes and the mixture is thereby cooled to a lower temperature. Liquid coolant is separated from the flash vapor and then flows through a heat exchanger wherein the liquid evaporates substantially at constant pressure as it absorbs heat from the heat exchanger. The vaporized coolant and flash vapors are again compressed and condensed and recycled through the heat exchanger. The most common system utilizing this method is known as a "cascade" system in which two or more coolant fluids are arranged in series so that the one with the lowest boiling point is condensed through the refrigerating effect caused by the evaporation of the one next higher in boiling point, and so on, until the one of highest boiling point is condensed by the atmosphere or by cooling water. One such system used for liquefaction and separation of air into its constituents has utilized three coolant fluids, ammonia, ethylene and methane.
The Claude cycle (iii) utilizes a combined expansion and heat exchange process. Compressed gas is cooled and thereafter expanded, whereby it is further cooled (and may be partially liquefied). The expanded stream is re-circulated through a heat exchanger to provide refrigeration requirements.